In convention examples relating to material for a wiring portion of a thermal flow measurement device comprising a heater and temperature detecting means disposed on both sides of the heater, as in a thermal air flow sensor and internal combustion engine control device described in Patent Document 1 mentioned below, a metal having a low melting point, such as aluminum or gold, is used for such wiring portion. Alternatively, polysilicon is used for the wiring portion, as in a thermal air flow sensor and internal combustion engine control device also described in Patent Document 1. As another alternative, a diffused resistor is used for the wiring portion, as in a semiconductor sensor device described in Document 2 mentioned below.
Examples of Patent Documents relating to the above technology are listed below:
Patent Document 1: JP Patent Publication (Kokai) No. 2002-48616 A
Patent Document 2: JP Patent Publication (Kokai) No. 2004-279038 A
When the thermal flow measurement device comprising a heater and temperature detecting means disposed on both sides of the heater is used as a flow measurement device for measuring engine intake air quantity, the environment in which the flow measurement device is disposed is very severe. This is because, in order to allow the thermal flow measurement device to measure air flow quantity, the engine intake air flow needs to come into direct contact with the surface of the detection element of the thermal flow measurement device. Conventional examples using metals having low melting points, such as aluminum or gold, for wiring portions lack consideration with respect to such point.
Engine intake air contains a great deal of dust, and the dust collides with the surface of the detection element of the thermal flow measurement device, resulting in destruction of the protective film on the surface of the detection element. In particular, when a metal material having a low melting point is used for the wiring material, the film quality of such protective film becomes poor and mechanically fragile. Further, an acid, such as sulfuric acid, nitric acid, or hydrochloric acid, contained in gasoline melts the protective film, and a metal such as aluminum corrodes when the acid reaches the inner metal wiring from the portion destroyed due to the above-described collision with dust.
On the other hand, conventional examples using polysilicon or diffused resistors for wiring portions are advantageous in terms of the above problems, since silicon has resistance to acid. However, since silicon has higher resistivity than metal, it also has higher wiring resistance. Such wiring resistance decreases the sensitivity of the thermal flow measurement device, and fluctuation in wiring resistance causes difference of characteristics. Still further, unnecessary heat generation is caused, thereby deteriorating the characteristics of the thermal flow measurement device. Further, silicon has a piezoresistive effect and a resistance value fluctuates with the influence of mounting stress, resulting in deterioration of characteristics.
The present invention has been made in view of the above circumstances, and it is an object of the present invention to provide a thermal flow measurement device that can suppress damage to sensor portions even in the above severe use environment and that can reduce wiring resistance.